The present invention is related to antistatic fabrics. More particularly the present invention relates to a system and method for decreasing electrostatic discharges to reduce the potential for incendiary discharges caused by electrostatic charges in flexible containers such as flexible intermediate bulk containers (FIBCs).
Containers formed of flexible fabric are being used in commerce more and more widely to carry free-flowable materials in bulk quantities. Flexible intermediate bulk containers have been utilized for a number of years to transport and deliver finely divided solids such as cement, fertilizers, salt, sugar, and barite, among others. Such bulk containers can in fact be utilized for transporting almost any type of free-flowable finely divided solid. The fabric from which they are generally constructed is a weave of a polyolefin, e.g., polypropylene, which may optionally receive a coating of a similar polyolefin on one or both sides of the fabric. If such a coating is applied, the fabric will be non-porous, while fabric without such coating will be porous. The usual configuration of such flexible bulk containers involves a rectilinear or cylindrical body having a wall, base, cover, and a closable spout secured to extend from the base or the top or both.
Such containers are handled by placing the forks of a forklift hoist through loops attached to the container. The weight of such a bulk container when loaded is typically between 500 pounds and 4,000 pounds, depending upon the density of the material being transported.
Crystalline (isotactic) polypropylene is a particularly useful material from which to fabricate monofilament, multifilament or flat tape yarns for use in the construction of such woven fabrics. In weaving fabrics of polypropylene, it is the practice to orient the yarns monoaxially, which may be of rectangular or circular cross-section. This is usually accomplished by hot-drawing, so as to irreversible stretch the yarns and thereby orient their molecular structure. Fabrics of this construction are exceptionally strong and stable as well as being light-weight.
Examples of textile fabrics of the type described above and flexible bulk containers made using such fabrics are disclosed in U.S. Pat. Nos. 3,470,928, 4,207,937, 4,362,199, and 4,643,119.
It has been found that the shifting of specific materials within containers made of woven fabrics, as well as particle separation between the materials and such containers during loading and unloading of the container cause triboelectrification and create an accumulation of static electricity on the container walls. In addition, the accumulation of static electricity is greater at lower relative humidity and increases as the relative humidity drops. Also, highly charged material entering such containers can create an accumulation of static electricity on the container walls. Electrostatic discharges from a charged container can be incendiary, i.e., cause combustion in dusty atmospheres or in flammable vapor atmospheres. Moreover, discharges can be quite uncomfortable to workers handling such containers.
One conventional approach to solving this problem is to use a grounded container. Such a container may include conductive fibers that are electrically connected to ground to carry the electric charge from the surface of the bag. The conductive yarns may be interconnected and one or more connection points may be provided for an external ground source. For example, Canadian Patent 1,143,673 and U.S. Pat. No. 4,431,310 disclose a fabric construction based on polyolefin yarn having conductive fibers in the yarns. Alternatively, the fabric may be coated with a layer of plastic film having an outer metalized surface, such as disclosed in U.S. Pat. No. 4,833,088.
The use of a grounded container, however, works only as long as the container remains grounded. If the container becomes ungrounded, its ability to decrease the potential for an incendiary discharge is lost, and due to the higher capacitance of the conductive system, the discharge can be much more energetic and incendiary than conventional non-conductive containers. Specifically, if such a container is not grounded, a spark discharge may develop which is capable of igniting flammable vapors or dust clouds and therefore must be grounded during the fill and emptying operations to provide a path for electrical discharge. Additionally, fabrication of the conductive containers requires specialized construction techniques to ensure all conductive surfaces are electrically connected together for a ground source.
Another conventional approach to decreasing the potential for incendiary discharges in flexible containers has been directed toward decreasing the surface electrostatic field of the container. If the magnitude of the electrostatic field on the surface of a container is above a certain threshold level, the potential for an incendiary discharge due to the electrostatic charge exists. That threshold level is about -500 kilovolts per meter (kV/m) for intermediate bulk containers made from woven polypropylene fabric. By decreasing the surface electrostatic field below about -500 kV/m, the potential for an incendiary discharge is greatly decreased and believed to be rendered virtually non-existent. Attempts at reducing the surface electrostatic field level below about -500 kV/m have not, however, proven successful without proper grounding.
One such effort at decreasing surface electrostatic fields has focused on the creation of corona discharges. There are four basic types of electrostatic discharges: spark discharges; brush discharges; propagating brush discharges; and, corona discharges. Of the four electrostatic discharges, the spark, the brush and the propagating brush electrostatic discharges can all create incendiary discharges. The corona discharge is not known to create incendiary discharges for common flammable atmospheres.
By incorporating certain materials into the flexible fabric container, as the electrostatic field increases, corona discharges from such materials limit the maximum field. This electrostatic field level, however, is above the -500 kV/m threshold level at which the potential for incendiary discharge first appears. Examples of this conventional approach include U.S. Pat. No. 4,207,376 (Nagayasu), U.S. Pat. No. 4,989,995 (Rubenstein), U.S. Pat. No. 4,900,495 (Lin), U.S. Pat. No. 4,997,712 (Lin), U.S. Pat. No. 5,116,681 (Lin) and U.S. Pat. No. 5,147,704 (Lin).
Yet another approach to the problem of incendiary discharge has been to decrease the surface resistivity of a container by coating the container with an antistatic material. Such a coating on the container surface increases the threshold level of the potential for an incendiary discharge to about -1500 kV/m. However, the potential for an incendiary discharge is still a very real possibility. Examples of this approach include U.S. Pat. No. 5,151,321 (Reeves) and U.S. Pat. No. 5,092,683 (Wurr).